Precision instrumentation systems typically require operational amplifiers (opamps) characterized by low noise, low offset, low offset drift, and low distortion in a frequency band of interest. One particular opamp design that meets these requirements is the multipath feed-forward opamp. This design provides high loop gain in a given frequency band and a very low corresponding unity gain crossover frequency. In particular, the high open loop gain minimizes distortion in the output signal. Furthermore, using relatively simple amplifier stages, conditionally stable multipath feed-forward amplifiers can be constructed which support high current loads (capacitive or resistive) while still maintaining stability and high output linearity.
The typical multipath feed-forward opamp is based on a cascade of integrator stages, all of which contribute to the open loop gain at low frequencies. At high frequencies, only the last stage, which is designed to have the highest bandwidth, remains in the signal path. During the transition from low to high frequencies, the integrator stages are bypassed, one at a time, until only the final stage is driving the output.
Depending on the particular multipath feed-forward opamp design, certain integrator stages in the cascade receive multiple input signals. These input signals may come, for example, from one or more prior stages in the cascade, or directly from the opamp external input. Those integrator stages that receive multiple input signals then internally sum the received signals with an integral summer circuit.
One significant problem found in present multipath feed-forward opamps comes from variations in the common mode voltage at the opamp external input. Specifically, inputs of those integrator stages that are directly coupled to the opamp external input are exposed to any variations in common mode voltage. The result is increased complexity in the design and/or reduced input voltage swing. Additionally, an increase in power consumption may result.
In multipath feed-forward amplifiers used in precision applications, such as instrumentation systems, minimal output signal distortion is a critical operational characteristic. For a given power consumption level, a maximum signal to distortion ratio in the frequency band of interest is a highly desired feature. To this end, the problem of minimizing the effects of variations in the common mode voltage at the inputs to the integrator stages must be addressed.